Patent Application
20150017469
The invention relates to a sheet steel for use as packaging steel according to the generic part of Claim 1 and a method for producing a packaging steel from a cold-rolled sheet steel according to the generic part of Claim 2.
Increasingly higher demands are made on the properties of metal materials for making packaging, in particular with regard to their formability and strength. Indeed, so-called dual-phase steels are known from automobile manufacturing; they have a multiphase structure that substantially consists of martensite and ferrite or bainite, and they on the one hand have high tensile strength and on the other also have high elongation at break. One such dual-phase steel with a yield point of at least 580 MPa and an elongation at break A80 of at least 10% is known, for example, from WO 2009/021898 A1. Because of the combination of the material properties of such dual-phase steels and high strength and good formability the said dual-phase steels are especially suitable for making complexly shaped and highly stressed components such as are needed, for example, in the area of automobile chassis construction.
As a rule, the alloying of the known dual-phase steels is composed of a martensite fraction of 20% to 70% and any residual austenite fraction and also ferrite and/or bainite. The good formability of dual-phase steels is ensured through a relatively soft ferrite phase and the high strength is produced by the solid martensite and bainite phases, which are bound into a ferrite matrix. In the case of dual-phase steels the desired properties with regard to formability and strength can be controlled in wide ranges through the alloy composition. For example, by adding silicon the strength can be increased by hardening the ferrite or bainite. Martensite formation can be positively affected through the addition of manganese and the development of perlite can be prevented. The strength can be increased by alloying with aluminum, titanium, and boron. Moreover, alloying with aluminum is used for deoxidation and for binding nitrogen that may be present in the steel. To develop the multiphase alloy structure dual-phase steels are subjected to a recrystallizing (or austenitizing) heat treatment, in which the steel strip is heated to sufficient temperatures and then cooled so that the desired multiphase alloyed structure is established with a substantially ferritic-martensitic structural development. For economic reasons cold rolled steel strips are usually recrystallization annealed in a continuous annealing process in an annealing furnace, where the parameters of the annealing furnace, for example pass-through rate, annealing temperature, and cooling rate, are established in correspondence with the required structure and the desired material properties.
A higher strength dual-phase steel and a method for its production are known from DE 10 2006 054 300 A1, where in the production process a cold- or hot-rolled steel strip is subjected to a continuous recrystallization annealing in a continuous annealing furnace in a temperature range of 820° C. to 1000° C. and the annealed steel strip is then cooled from the said annealing temperature at a cooling rate between 15 and 30° C. per second.
Dual-phase steels known from automobile manufacturing are, as a rule, not suitable for use as packaging steel, since they are very expensive, particularly because of the high fractions of alloying elements like manganese, silicon, chromium, and aluminum and since, for example, some of the known alloying elements must not be used for packaging steel in the food area, since contamination of foods by diffusion of alloy components into the packaging contents must be excluded. In addition, many of the known dual-phase steels have strengths so high that they cannot be cold-rolled with the systems that are usually used for production of packaging steel.
Proceeding from this, the invention is based on the task of making available a higher strength steel with good formability for use as packaging steel that is as cost effective as possible to produce. Further, the invention is intended to point out a method for production of a packaging steel that can be made cheaply and that has high strength and high elongation at break.
These tasks are solved with a sheet steel having the features of Claim 1 and with a method having the features of Claim 2. Preferred embodiment examples of the sheet steel and the method for making it are indicated in the dependent claims.
The sheet steel according to the invention for use as packaging steel is made from a low-alloy and cold-rolled steel with a carbon content of less than 0.1%. When used below, the term “sheet steel” will be understood to mean such a steel. Besides the low carbon content and low concentrations of the other alloy components, the sheet steel according to the invention is characterized by a multiphase structure, which comprises ferrite and at least one of the structural components martensite or bainite. The steel from which the sheet steel according to the invention is made can be a cold-rolled unalloyed or low alloy steel. Steels in which no alloy element exceeds an average content of 5% are called low alloy steels. The steel used to make the sheet steel according to the invention in particular has less than 0.5 wt % and preferably less than 0.4 wt % manganese, less than 0.04 wt % silicon, less than 0.1 wt % aluminum, and less than 0.1 wt % chromium. The steel can contain alloying additions of boron and/or niobium and/or titanium in order to increase the strength, where the alloying with boron expediently lies in the range of 0.001-0.005 wt % and the alloying with niobium or titanium lies in the range of 0.005-0.05 wt %. However, weight fractions that are <0.03% are preferred for Nb.
To develop the multiphase alloy structure the steel for making the sheet steel according to the invention for use as packaging, steel is first subjected to recrystallization annealing by electromagnetic induction at a heating rate of more than 75 K/s and cooled after the recrystallizing induction annealing at a cooling rate of at least 100 K/s. Through the recrystallizing heat treatment (with Tmax>Ac1, since austenitization is necessary) and subsequent rapid cooling there forms the multiphase structure, which comprises ferrite and at least one of the structural components martensite, bainite, and/or residual austenite. The sheet steel treated in this way has a tensile strength of at least 500 MPa and an elongation at break of more than 6%.
The recrystallizing (or austenitizing) annealing of the sheet steel by means of electromagnetic conduction proved to be an especially important parameter for the production of the packaging steel according to the invention. It was surprisingly established that the alloying of alloy components that are typically contained in dual-phase steels, for example the alloying of manganese (which typically has a weight fraction of 0.8-2.0% in the known dual-phase steels), silicon (which typically has a weight fraction of 0.1-0.5% in the known dual-phase steels), and aluminum (which is alloyed in a weight fraction up to 0.2% in the known dual-phase steels) can be omitted if a cold rolled sheet steel with carbon content less than 0.1 wt % is first subjected to recrystallization (or austenitizing) annealing at a heating rate of more than 75 K/s by means of electromagnetic induction and then quenched at a high cooling rate of at least 100 K/s.
The surprising effect of the inductive heating on the development and arrangement of the martensite phase in the induction annealed steel strip may be explained as follows: ferromagnetic substances are not magnetized in the absence of an external magnetic field. However, within these substances there are regions (Weiss regions), which are magnetized to saturation even in the absence of external magnetic fields. The Weiss regions are separated by Bloch walls. Through the application of an external magnetic field first the favorably oriented, thus energetically preferred, Weiss regions grow at the expense of adjacent regions. The Bloch walls shift as this occurs. The electronic spin flip in this case does not take place everywhere simultaneously, rather the spins change direction at the boundaries of the Weiss regions first. With a further increase of the field, the direction of magnetization rotates into that of the field until the spins correspond in all of the regions with that of the external magnetic field and saturation is reached. It is also known that a magnetic field can affect the motion of dislocations in the absence of externally applied mechanical stresses. It now seems plausible that the Bloch walls entrain carbon atoms and/or dislocations when they shift. Through this carbon and/or dislocations collect in certain areas, in which martensite forms after annealing and quenching.
Expediently, the sheet steel according to the invention for use as packaging steel is fine or ultrafine sheet that was rolled to its end thickness in a cold rolling process. Fine sheet is understood to mean a sheet with a thickness of less than 3 mm and an ultrafine sheet has a thickness of less than 0.5 mm. After the recrystallization annealing and cooling, the sheet steel can be provided with a metal surface coating, for example of tin, chromium, aluminum, zinc, or zinc/nickel, to increase its corrosion resistance. The known electrolytic coating processes, for example, are suggested for this.
The invention is explained in more detail below by means of an embodiment example:
To produce embodiment examples of the sheet steel according to the invention, steel strips of steels having the following composition [were made], which were made in a continuous casting process and hot rolled and wound into coils:
This sheet steel was first cold rolled with a thickness reduction of 50% to 96% to an end thickness in the range of about 0.5 mm and then recrystallizing-annealed in an induction furnace under induction heating. For example, an induction coil with 50 kW power at a frequency of f=200 kHz was used for a sample size of 20×30 [sic]. The annealing curve is shown in
The thus treated sheet steel was then tested with regard to strength and elongation at break. It was shown by comparison experiments that in all cases the elongation at break was higher than 6% and as a rule higher than 10% and that the tensile strength was at least 500 MPa and in many cases even turned out to be more than 650 MPa.
It was shown by a color etching following Klemm that the sheet steel treated according to the invention has an alloy structure that has ferrite as the soft phase and martensite and possibly bainite and/or residual austenite as hard phase.
It was determined by comparison experiments that the best results with regard to strength-formability are achieved if the heating rate in the recrystallizing induction annealing lies between 200 K/s and 1200 K/s and when the recrystallizing-annealed steel strip is then cooled at a cooling rate of more than 100 K/s. Expedient from the standpoint of equipment here are cooling rates between 350 K/s and 1000 K/s, since in this case a costly water cooling can be omitted and cooling can take place by means of a cooling gas such as air. To be sure, the best results with regard to the material properties are achieved when using water cooling at cooling rates of more than 1000 K/s.
The sheet steel according to the invention is outstandingly suitable for use as packaging steel. For example, cans for food or beverages can be made from the sheet steel according to the invention. Since higher demands are made on the corrosion resistance of packaging in particular in the food field, it is expedient to provide the sheet steel produced according to the invention with a metallic and corrosion resistant coating after the heat treatment and possibly after a final dressing or a cold rolling step, for example by electrolytic tin plating or chrome plating. However, other coating materials such as aluminum, zinc, or zinc/nickel, and other coating methods, for example hot dip zinc plating, can also be used. In each case according to requirements the coating can take place on one side or both sides.
Compared to the dual-phase steels known from the automobile construction the sheet steel according to the invention for use as packaging steel is characterized in particular by the considerably lower production costs and by the advantage that a steel with lower alloy concentration and fewer alloy components can be used, so that contamination of the packaged foods can be avoided. With regard to the strength and formability, the sheet steel according to the invention is comparable to the dual-phase steels known from automobile construction.
| Number | Date | Country | Kind |
|---|---|---|---|
| 10 2011 056 847.6 | Dec 2011 | DE | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/EP2012/069465 | 10/2/2012 | WO | 00 |
